The present invention relates to a process for the phosgenation of amines in the gas phase, in which a specific type of heat exchanger is used for vaporizing the amines.
EP-A 0 289 840 describes a process for preparing (cyclo)aliphatic diisocyanates by phosgenation of the corresponding gaseous (cyclo)aliphatic diamines at from 200° C. to 600° C. Phosgene is introduced in a stoichiometric excess. The superheated streams of gaseous (cyclo)aliphatic diamine or (cyclo)aliphatic diamine/inert gas mixture and of phosgene are introduced continuously into a cylindrical reaction space and mixed with one another and reacted there. The exothermic phosgenation reaction is carried out while maintaining turbulent flow.
EP-A 928 785; EP-A 1 319 655; EP-A 1 555 258; EP-A 1 275 639; EP-A 1 275 640; EP-A 1 403 248; and EP-A 1 526 129 each describes a specific embodiment of this technology, but these disclosures relate to the reactor itself and the reaction conditions without going into details about the vaporizer technology used for pre-treatment of the starting materials.
Shell-and-tube heat exchangers, plate heat exchangers or falling film evaporators, preferably with a pumped circuit, are customarily used for heating and vaporizing the starting materials used, i.e., amines and phosgene. Heater coils matrices operated electrically or by means of heat transfer fluids are used for heating the gaseous amines. However, these apparatuses have the disadvantage that the relatively high film thicknesses which occur adversely affect mass transfer and heat transfer, so that an increased residence time is required. As a result, decomposition with elimination of ammonia occurs, particularly in the vaporization and superheating of aliphatic amines. This not only reduces the yield but also causes the formation of deposits of ammonium chloride in pipes and the reactor in the subsequent phosgenation reaction. The plants therefore have to be cleaned relatively frequently, resulting in corresponding losses of production.
Micro heat exchangers or micro vaporizers have been described in WO 2005/016512 but only in the context of removal of compounds from liquid mixtures by distillation. However, in the field of gas-phase phosgenation of amines to form isocyanates, these apparatuses have not been described in any respect nor have their possible advantages been mentioned.